Results and Discussions Chapter-5 The detailed numerical values of the mass fraction changes with respect to variable operating conditions are presented in Table 5.6.
Results and Discussions Chapter-5
and aromatics from the bio-oil is calculated by the following expression. The major contribution of alkane and aromatics production is through the conversion of heavy non-volatiles whereas phenols are converted very little to the desired alkane and aromatics as per the reaction mechanism considered in the present simulation study (See Figure 3.2).
Figure 5.28 compares the yield of alkane and aromatic from the heavy non-volatile residue and phenol fractions of bio-oil at longer residence time 2 h-1 at different operating temperatures, and pressures in the presence of three different catalysts namely (Pt/Al2O3, Ni- Mo/Al2O3, Co-Mo/Al2O3). At a fixed temperature of T=623 K and at higher pressures P=10443 kPa, the yield of alkane and aromatic with respect to catalyst activity is in the following order as Co-Mo/Al2O3 > Pt/Al2O3 > Ni-Mo/Al2O3; however, Pt/Al2O3 catalyst exhibit higher yields of alkane and aromatics at lower pressures for a fixed range of temperature. Similarly with the increasing temperature and constant low pressure (P=6996 kPa) Pt/Al2O3 yields higher values of alkane and aromatics as compared to other catalysts. It is also observed that the yields of alkane and aromatics in the presence of Ni-Mo/Al2O3 catalysts tends to increase gradually with the increasing pressure, but higher values are reported for the moderate range of temperatures T=648 K. In the case of Co-Mo/Al2O3 catalyst the values are consistently showing an increasing trend with respect to temperature, and pressure as compared to other two catalysts reporting the higher activity irrespective of the operating conditions. time WHSV=3h-1 in the presence of different catalysts at various operating conditions. It is observed from the figure that the formation of alkane and aromatic from the reactants (HNV, Phenol) is almost unchanged with respect to operating conditions in the case of Pt/Al2O3; however minute fluctuations are seen in the case of Ni-Mo/Al2O3 catalyst whereas Co-Mo/Al2O3 catalyst possess an increasing trend with the increasing pressure.
Results and Discussions Chapter-5
Figure 5.28: Yield of alkane and aromatic at WHSV=2 h-1for different operating conditions Figure 5.29 elucidate the yield of alkane and aromatic formation at moderate residence conditions in the case of Pt/Al2O3; however minute fluctuations are seen in the case of Ni- Mo/Al2O3 catalyst whereas Co-Mo/Al2O3 catalyst possess an increasing trend with the increasing pressure. As the residence time is reduced the formation of alkane and aromatics is significant in the case of Co-Mo/Al2O3 due to effect of the hydrogen pressure and higher temperatures reducing the coke formation tendency and converting the reactants to value added products. The overall yield of the alkane and aromatics has been increased from 45% to 50% with reducing residence time. Further shortening the residence time to 4 h-1 the product formation is still enhanced to 60% in the case of Co-Mo/Al2O3, followed by Pt/Al2O3 and Ni-Mo/Al2O3
catalysts.The lower residence times avoids the secondary reactions and also the formation of the secondary products. The secondary products are formed due to the prolonged reactions at such a high temperatures and pressures further leads to the blockage of acid sites of the catalysts or deactivations of the catalysts. On the other hand, Ni-Mo/Al2O3 catalyst tends to be more reactive at shorter residence times and hence the yield of alkane and aromatics is almost equal to the yield
Results and Discussions Chapter-5
Figure 5.29: Yield of alkane and aromatic at WHSV=3 h-1for different operating conditions of alkane and aromatics through Pt/Al2O3 catalyst at P=6996 kPa and T=623 K as seen in Figure 5.30. Higher yields of alkane and aromatic via Ni-Mo/Al2O3 catalyst are possible with the lower pressures, lower temperatures and lower residence times. Pt/Al2O3 catalyst again follows the similar trend of constant yield of 20% alkane and aromatics at shorter residence time.
Figure 5.30: Yield of alkane and aromatic at WHSV=4 h-1for different operating conditions
Conclusions Chapter-6
The effect of catalysts on the hydrodeoxygenation of bio-oil in an ebullated bed reactor is studied through CFD simulations. The aim was to identify the effects of temperature, pressure, and catalyst loading on the hydrodynamics of the process and to incorporate the chemical reaction kinetics in a numerical solver. Some of the key conclusions are given below.
6.1. Effect of catalyst load
The volume fraction contours of the three phases (bio-oil, H2 gas and solid catalyst phases) varying with the catalyst load i.e., WHSV is presented in detail. The attainment of the steady state of catalyst bed is observed at t=80 sec for all the three catalysts namely Pt/Al2O3,Ni- Mo/Al2O3, and Co-Mo/Al2O3 catalysts.
For Pt/Al2O3, catalyst, it is observed that the increasing values of WHSV do not show any impact on the volume fraction of the catalyst phase irrespective of temperature and pressure.
Similarly, the H2 gas phase also reported the constant trend i.e., without much deviation with the increasing values of WHSV at all operating conditions. But, the fluctuations in the bio-oil phase is noted at the initial level i.e., at lower values of WHSV=2 h-1 and higher temperature and becomes linear for all the values of temperature and pressure; however the low and moderate temperature curves are not affected by the operating variables. In the case of Ni-Mo/Al2O3
catalyst, the catalyst and gas volume fractions doesn’t deviate and follows a linear trend with the increasing values of WHSV at respective operating conditions of temperature and pressure.
While, small deviations/fluctuations are seen in the case of oil volume fractions with the changing WHSV values. Finally for the Co-Mo/Al2O3 catalyst, the volume fraction of the gas phase is increasing continuously with the increasing WHSV. The reverse trend is clearly seen for the bio-oil phase, while the catalyst phase reported slighter reduction
Conclusions Chapter-6
in the values with the increasing WHSV. At higher values of WHSV all the three phases are stable with no deviations or changes further.
On the other hand the chemical reaction kinetics reported significant changes with the change in the WHSV values. As the value of the residence time is decreased the reaction is incomplete which reduces the formation of the desired product species. The effect of the residence time is also dependent on the reaction rates, catalyst activity, and reaction mechanisms i.e., some catalyst requires higher residence times for the completion of reaction to form desired products. While some catalyst possesses the catalytic inactivity due to the higher residence times due to the formation of secondary products which are cracked from the primary desired compounds. Hence, the optimized condition of WHSV is to be chosen for the higher quantity and quality of the desired products. It is observed that the non-volatile fractions of the bio-oil show minimal variations regardless of the WHSV values of Pt/Al2O3 catalysts. The phenols formation from the non-volatiles or the conversion of phenols to alkane and aromatics are almost constant at all WHSV values. The formation of the alkane and aromatics from the heavy non- volatile residues and the phenols tend to be higher at lower values of WHSV. In the case of Ni- Mo/Al2O3 catalyst the conversion of the non-volatile fractions to phenols are on higher note instead of forming alkane and aromatics. The major observation here is the phenol formation is higher at lower residence time as the activity of the catalyst tends to be very much higher at lower residence times. Similarly, the alkane and aromatics formation is also considerably higher at lower residence times. Further in the case of Co-Mo/Al2O3 catalyst as the values of the WHSV is increasing the conversion of the non-volatile fractions tends to increase and reported higher conversions at higher WHSV. The obtained results suggest that the decreasing trend of non- volatile fractions represents the higher conversions of non-volatiles to desired products. This
Conclusions Chapter-6
phenomena is observed majorly at the lower values of the WHSV i.e., higher residence times.
Thus, the increase of space velocity shortens the reactants residence time in the catalyst bed, leading to decrease hydrodeoxygenation, cracking, gasification and other secondary processes such as removal of oxygenates from the unprocessed bio-oil.